Anion-Directed Coordination Networks of a Flexible S-Pivot Ligand

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Anion-Directed Coordination Networks of a Flexible S‑Pivot Ligand and Anion Exchange in the Solid State Eunji Lee, Huiyeong Ju, Seulgi Kim, Ki-Min Park,* and Shim Sung Lee* Department of Chemistry and Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea S Supporting Information *

ABSTRACT: Using an S-pivot joint flexible ligand bis(4-cyanobenzyl)sulfide (bcbs), three silver(I) complexes [Ag(bcbs)ClO4]n (1), [Ag2(bcbs) (CF3CO2)2]n (2), and {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n (3) have been isolated. Configurations (“V”-, “L”-, and “W”-shapes) of the bcbs ligand in 1−3 can be tuned as a result of changing the anions of the silver(I) salts, which ultimately form a series of infinite low to high dimensional frameworks with different structures. The perchlorato compound 1 shows a stairway type 1-D ribbon structure, in which the anion acts as a monodentate terminal ligand. The bcbs ligand in 1 shows a gauche−anti arrangement, adopting an L-shape configuration. The π−π stacking interaction between adjacent ribbons in 1 yields a pseudo-2-D structure with a herringbone pattern. Compound 2 is composed of silver(I)−bcbs helical channels, which are further interlinked via monodentate bridging CF3CO2− ions forming a 3-D multichannel framework with a dmd topology. Unlike 1, the bcbs ligand in 2 shows an anti−anti arrangement, adopting a W-shape configuration. The corresponding PF6− complex 3 exhibits a 2-fold interpenetrated 3-D framework. The preferred anion-free structure in 3 presumably reflects the strong coordination of the donor set and the weak affinity of the PF6− toward the metal center. Similar to 1, the bcbs ligand in 3 adopts an L-shape configuration. In this work, the structural variations of the resulting coordination polymers are mainly associated with the diverse configurations of the flexible ligand, as well as the different coordination modes and abilities for the anions. Anion exchange accompanying an irreversible structural conversion from 2 or 3 to 1 was monitored in the crystalline state by IR and PXRD studies. AFM images and height profiles for the crystal surface before and after the anion-exchange were observed. On the basis of the above measurements, a mechanistic process from the 3-D structured 3 (PF6− form) to the 1-D structured 1 (ClO4− form) by the anion exchange was also proposed.

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INTRODUCTION The construction of supramolecular coordination polymers highly depends on the subtle balance between multidentate ligands and coordination properties of metal ions.1−6 However, the frameworks or network structures based on flexible ligands are less predictable because the conformational diversity of the ligands leads to a variation that influences the topological architectures which are largely considered to be serendipitous.7−11 In fact, it is realized that access to other topologies is achieved by incorporating some flexibility into the ligand in the form of conformationally free subunits, such as −CH2−X− CH2− type short chains.12−14 So far, many kinds of “X−(CH2− arene)2” type (X = N or O) tridentate flexible ligands have been reported for the complexation with metal ions.15−21 On the contrary, there are few examples of the S-donor containing flexible analogues.22−25 We have long been involved in the synthesis of S-containing cyclic26−36 and acyclic22−25 ligands and their metallosupramolecules. It is expected that the cooperation between sulfur donor and nitrogen atoms might bring a new aspect in the metal complexation to demonstrate new types of supramolecular frameworks. So, we have interested in the construction of new coordination frameworks using the flexible tridentate ligands with an “S−(CH2−arene)2” arrangement in © XXXX American Chemical Society

which the terminal arene derivatives as a binding site are separated by a three-atomic thioether spacer. Since these ligands have to be derivatized, the system we have studied possesses 4-substituted pyridine donors [bis(4-pyridylmethyl)sulfide] (4-bpms) 23,24 or cyanophenyl groups [bis(4cyanobenzyl)sulfide (abbreviated as bcbs)] as the aromatic terminals. Recently, we have reported a unique triple-stranded ladder type 2-D Ag(I) coordination networks constructed from 4-bpms using a stepwise approach starting from the 1-D network.23 Since the sulfur atom in bcbs ligand can act not only as a binding site for the soft metals but also as a pivot joint which influences to the conformational change of the ligand. As we understand, the synthesis of bcbs was reported previously,37,38 however, the no complexes of this compound have been reported so far. More recently, we have reported the synthesis of its 2- and 3-substituted regio-isomers and their silver(I) nitrate coordination polymers.22 In the complexation products of the 3-bcbs, interestingly, molar ratio dependent supramolecular isomerism has been observed due to the conformaReceived: July 23, 2015 Revised: October 1, 2015

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DOI: 10.1021/acs.cgd.5b01050 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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shows nonplanarity with a twisted conformation (∠C−S1−C 101.64°) and a large dihedral angle (39.95°) (Figure 1 and

tional changes of the ligand. The bcbs ligand can possibly show three configurations due to the rotation of C−C and C−S bonds which differ in the torsion angles between the two aromatic rings (Scheme 1). These versatile ligand configurations can give rise to the construction of coordination frameworks with different structures.39−44 Scheme 1. Representative Configurations of bcbs as an SPivot Ligand

Figure 1. Crystal structure of bcbs ligand with a gauche−gauche configuration (V-shape in Scheme 1, torsion angles for C5−C8−S1− C9 71.2° and C10−C9−S1−C8 74.9°).

Table 1). The flexible aliphatic thioether segment, C5−C8− S1−C9−C10, shows a bent arrangement with a gauche−gauche configuration (or V-shape, see Scheme 1a) of the torsion angles for C5−C8−S1−C9 and C8−S1−C9−C10 units). The distance between N1 and N2 in bcbs is 12.88 Å. Preparation and Structures of Silver(I) Complexes with Different Anions (1−3). In an attempt to investigate the effect of anion variations on the silver(I) complexations toward bcbs, three silver(I) salts, such as AgClO4, AgCF3CO2, and AgPF6, were employed. L dissolved in dichloromethane was treated with one equivalent of each silver(I) salt in methanol. Slow evaporation of the solutions at room temperature afforded colorless crystalline products of 1 (ClO4− form), 2 (CF3COO− form), and 3 (PF6− form) (Scheme 2). The X-ray analysis reveals that all the complexes are polymeric species of types [Ag(bcbs)ClO4]n (1), [Ag2(bcbs) (CF3CO2)2] (2), and {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n (3) whose structures and dimensionalities are dependent to the anions which induce the conformational changes of bcbs ligand (Table 1 and Figures 2−4). The selected geometric parameters are presented in Tables 2−4. Coordination polymers 1−3 are stable in air and insoluble in common organic solvents. The IR spectra for all of the complexes show strong absorption bands in the region, 2248−2253 cm−1 that indicate coordinated cyano groups.45−48 Reaction of AgClO4 with bcbs, a stairway (or zigzag ribbon) like one-dimensional (1-D) polymer species of type [Ag(bcbs)ClO4]n (1) was isolated (Figure 2a). Selected structural parameters are listed in Table 2. The complex 1 crystallizes in the triclinic space group P1̅ with Z = 2 where the asymmetric unit contains one Ag atom, one bcbs ligand, and one ClO4−. As might be expected, two cyano N atoms from two different bcbs ligands in one strand bind strongly to the silver(I) center [Ag1−N1A 2.311(2) Å, Ag1−N1B 2.224(2) Å] (Figure 2b). And one sulfur donor from the adjacent parallel strand binds to the silver(I) center [Ag1−S1 2.493(1) Å], resulting in the cross-linked double-stranded chain forming the stairway structure. The coordination sphere around the silver(I) center is completed by a monodentate ClO4− with a bond length [Ag1−O1 2.534(2) Å] that falls within the normal range.49−51 Such silver(I) coordination in 1 also generates the simple rectangular metallacycle subunit in which two Ag atoms are doubly linked by two of a half-molecule of ligand via Ag−S and Ag−N bonds. The coordination geometry around the Ag atom is distorted tetrahedral with the “tetrahedral” angles in the range 87.3(1)−129.2(1)°. The tridentate bcbs ligand which binds to three Ag atoms shows a gauche−anti arrangement. So, the two consecutive rectangular block units contain one bcbs ligand with an “L”-shape (Scheme 1b and Figure 2c) and the length of bcbs (N1···N2 12.83 Å) in 1 is similar to that of the

In this study, as an extension of our ongoing work on the supramolecular coordination polymers with the sulfur donor ligands,22−36 the preparation and solid structures of the free bcbs ligand and its anion-dependent 1-D to 3-D silver(I) coordination polymers [Ag(bcbs)ClO4]n (1), [Ag2(bcbs) (CF3CO2)2]n (2), and {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n (3) in which the three different ligand configurations (Scheme 1) can be observed. The free volumes in these structures were calculated to be less than 2% which seems to be responsible for the flexible nature of the ligand. In this work therefore, instead of the gas absorption experiment, the structural conversion of the products via anion-exchange as postsynthetic procedures was accomplished as summarized in Scheme 2. Scheme 2. Anion-Dependent Synthesis of Silver(I) Coordination Polymers with bcbs and Their Structural Conversion via Anion-Exchange

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RESULTS AND DISCUSSION Crystal Structure of bcbs Ligand. The bcbs ligand was prepared from the reaction of p-cyanobenzylmerchaptan and pcyanobenzyl chloride as reported previously.37,38 Since no solid structure of bcbs has been reported, its single crystals suitable for the X-ray analysis were grown by slow evaporation from the dichloromethane solution. In crystal, overall structure of bcbs B



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Table 1. Crystal and Experimental Data formula formula weight temperature crystal system space group Z a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Dcalcd (g/cm3) 2θmax (deg) R1, R2 [I > 2σ(I)] R1, R2 [all data] no. of reflns used [>2σ(I)] refinement

bcbs

1

2

3

C16H12N2S1 264.34 173(2) orthorhombic Pna2(1) 4 12.2259(5) 23.8611(11) 4.6470(2) 90 90 90 1355.64(10) 1.295 56.68 0.0406, 0.0859 0.0531, 0.0914 3382 [R(int) = 0.0565] full-matrix

C16H12AgClN2O4S 471.66 173(2) triclinic P1̅ 2 7.2575(3) 8.1541(3) 16.1939(6) 103.278(2) 90.896(2) 114.302(2) 843.48(6) 1.857 56.00 0.0243, 0.0598 0.0267, 0.0608 4080 [R(int) = 0.0275] full-matrix

C20H12Ag2F6N2O4S 706.12 173(2) monoclinic C2/c 4 23.2660(6) 8.0180(2) 15.5880(5) 90 130.161(2) 90 2222.31(11) 2.110 56.00 0.0176, 0.0459 0.0193, 0.0466 2691 [R(int) = 0.0307] full-matrix

C68H56Ag3Cl8F18N8P3S4 2155.57 173(2) monoclinic P2/c 2 20.9049(4) 14.6808(3) 14.2515(3) 90 108.0430(10) 90 4158.71(15) 1.721 56.00 0.0394, 0.0903 0.0587, 0.1012 10052 [R(int) = 0.0518] full-matrix

Figure 2. 1-D silver(I) perchlorate coordination polymer 1, [Ag(bcbs)ClO4]n: (a) zigzag ribbon-type polymeric structure showing interchain π−π stacking interaction (dotted lines), (b) core coordination unit, and (c) “L”-shape ligand configuration showing a gauche−anti arrangement for C− C−S−C−C unit. Coordinating anions in panels b and c are omitted.

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 1, [Ag(bcbs) (ClO4)]na Ag1−O1 Ag1−N1A O1−Ag1−S1 O1−Ag1−N2B S1−Ag1−N2B

2.534(2) 2.311(2) 98.4(1) 110.9(1) 129.2(1)

Ag1−S1 Ag1−N2B O1−Ag1−N1A S1−Ag1−N1A N1A−Ag1−N2B

Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) for 2, [Ag2(bcbs) (CF3CO2)2]na

2.493(1) 2.224(2) 87.3(1) 115.4(1) 106.9(1)

Ag1−N1C Ag1−O1A N1C−Ag1−O1 N1C−Ag1−S1 O1−Ag1−S1

2.262(2) 2.388(1) 120.4(1) 119.7(1) 107.6(1)

Ag1−O1 Ag1−S1 N1C−Ag1−O1A O1−Ag1−O1A O1A−Ag1−S1

2.369(1) 2.512(1) 98.9(1) 72.8(1) 130.2(1)

a

Symmetry operations: (A) −x + 2, −y, −z; (B) −x + 2, −y + 1, −z + 1.

Symmetry operations: (A) −x, −y + 1, −z; (C) −x + 1/2, −y + 1/2, −z + 1.

free form (12.88 Å, Figure 1). In packing, the zigzag ribbons in 1 are stacked with face-to-face π−π interactions52 to yield a

herringbone type pseudo 2-D structure (dashed lines in Figure 2a, centroid-to-centroid distance: 3.63 Å).

a

C



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four-coordinate, being bound to one S donor [Ag1−S1 2.512(1) Å] from one bcbs ligand and one cyano N atom [Ag1−N1C 2.262(2) Å] from another bcbs ligand. The coordination sphere is completed by two monodentate bridging μ-CF3CO2− ions forming an Ag1−(μ-O)2−Ag1A square [Ag1− O1 2.369(1), Ag1−O1A 2.388(1), Ag1···Ag1A 3.83 Å]. In 1, one ClO4− occupied one binding site in the tetrahedral four-coordinated Ag(I) as a monodentate terminal ligand. However, the CF3COO− in 2 occupies two positions as a bridging ligand due to the stronger coordination ability and smaller steric hindrance to form a higher dimensional structure. Unlike 1, the bcbs ligand in the helical channel shows an anti− anti arrangement (Figure 3d), adopting a W-shape configuration (Scheme 1c and Figure 3c). To understand the 3-D framework of 2, the topological approach is also applied to simplify the complicated structure. As mentioned, two Ag(I) atoms are combined together by two carboxylate bridges from two different CF3COO− to form a dinuclear Ag2(CF3CO2)2 square cluster. By considering Ag1 atom and S1 atom in the bcbs ligand as 3- and 4-connecting nodes, respectively, the overall topology belongs to a (3,4)-connected {4 × 102}2{42.104} dmd53−55 net (Figure 3e). When an equimolar amount of AgPF6 was employed in the reaction of bcbs, it afforded an infinite 2-fold interpenetrated 3D framework of type {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n (3) (Figure 4a and 4b). Selected structural parameters are listed in Table 4. Complex 3 crystallizes in the monoclinic space group P2/c with Z = 2 where the asymmetric unit of the complex part consists of two crystallographically independent Ag atoms (Ag1 having a half occupancy and Ag2 with full

Table 4. Selected Bond Lengths (Å) and Bond Angles (deg) for 3, {[Ag3(bcbs)4](PF6)3·4CH2Cl2}na Ag1−N2A Ag2−N1 Ag2−N4E N2A−Ag1−N2B N2B−Ag1−S1 S1−Ag1−S1C N1−Ag2−N4E N1−Ag2−S2 N4E−Ag2−S2

2.325(3) 2.282(3) 2.378(3) 96.9(2) 117.8(1) 117.0(1) 105.1(1) 129.1(1) 98.9(1)

Ag1−S1 Ag2−N3D Ag2−S2 N2A−Ag1−S1 N2B−Ag1−S1C N1−Ag2−N3D N3D−Ag2−N4E N3D−Ag2−S2

2.531(1) 2.293(3) 2.487(1) 103.1(1) 103.1(1) 92.7(1) 109.2(1) 120.7(1)

a Symmetry operations: (A) x, −y − 1, z − 1/2; (B) −x + 2, −y − 1, −z + 2; (C) −x + 2, y, −z + 3/2; (D) −x + 1, −y, −z + 1; (E) x, −y + 1, z + 1/2.

In order to investigate the effect of anion on the above reaction type, the same synthetic procedure was repeated employing CF3CO2− instead of ClO4− as the counteranion of silver(I) salt. In this case, bcbs forms a 3-D multichannel framework of type [Ag2(bcbs) (CF3CO2)2]n (2) (Figure 3a). Selected structural parameters are listed in Table 3. The singlecrystal structure analysis reveals that complex 2 crystallizes in monoclinic space group C2/c with Z = 4. The asymmetric unit contains one Ag(I) atom, half of bcbs ligand, and one CF3CO2−. In the dinuclear Ag2(CO2)2S2N2 secondary building unit (SBU) shown in Figure 3b, two halves of bcbs connect two Ag atoms to generate a looped chain with the sulfur nodes, adopting a helix-type channel structure which are further linked by two monodentate carboxylate groups resulting in the formation of the 3-D multichannel structure. The Ag atom is

Figure 3. 3-D silver(I) trifluoroacetate coordination polymer 2, [Ag2(bcbs) (CF3CO2)2]n: (a) top view of the multichannel structure linked by anions (terminal anions are omitted), (b) core coordination part containing Ag2(CO2)2S2N2 SBU, (c) W-shape ligand configuration showing an anti−anti arrangement for C−C−S−C−C unit, (d) side view of the multichannel linked by anions (terminal anions are omitted), and (e) view of the dmd topology. D



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Figure 4. 2-Fold interpenetrating 3-D silver(I) hexafluorophosphate coordination polymer 3, {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n: (a) perspective view of the 2-fold interpenetrating framework, (b) single framework, (c) core coordination part containing SBU, (d) “L”-shape ligand configuration showing the gauche−anti arrangement, (e) simplified 2-fold interpenetrating framework, and (f) simplified single framework showing the ssl1 topology. Noncoordinating anions and solvent molecules are omitted.

strong coordination of the donor set and the weak affinity of the PF6− for binding to this metal center. Thermal Properties. TGA analyses were performed to investigate the thermal stabilities of 2 and 3 (Figure 5). The perchlorate compound 1 has not been analyzed due to the potential for explosion at high temperature. The TGA curves revealed that polymeric products 2 and 3 are stable up to 200 and 220 °C, respectively. But upon further heating, the structures begin to decompose, presumably reflecting the loss of organic molecules. Anion-Exchange in the Crystalline State. Structural conversion of the anion-dependent products 1−3 with different dimensionalities was investigated by anion-exchange in the crystalline state. Since all of these products are found to be airstable and insoluble in water, the crystalline sample was immersed in a 3 M aqueous sodium salt solution of the required anion and left undisturbed at ambient temperature to allow anion-exchange. Since the crystals taken out from the solution were not diffracted because of their poor crystallinity,

occupancy) and two bcbs ligands. The Ag1 center is tetrahedrally four-coordinate, being bound to two S and two cyano N donors from four different ligands (Figure 4c). While, the Ag2 atom is bonded to one S and three cyano N donors from four bcbs ligands to form a distorted tetrahedral environment. As a result, each Ag center connects four bcbs ligands and every bcbs ligands connect four Ag(I) atoms. Similar to the case in 1, both of two bcbs ligands in one large rectangular block units in 3 show a gauche−anti arrangement, adopting an “L”-shape configuration again (Scheme 1b and Figure 4d). Both of the 4-connected Ag(I) atoms (Ag 1 and Ag2) and 3-connected sulfur atoms (S1 and S2) can be regarded as nodes, and the half molecule of the bcbs ligand can be regarded as a bridging linker in the construction of this network. It is this infinite extension of the ligand which results in a new (3,3,4,4)-connected net, which was analyzed using TOPOS56−60 and was named as ssl1 (Figure 4e and 4f), with point symbol {4·82}4{4·84·10}2{42·82·102}. The preferred anion-free 3-D structure in 3 presumably reflects the overall E



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To investigate the reversibility of the anion-exchange process between 1 and 3, the crystals of 1 was dipped in an aqueous solution of NaPF6. In Figure 7, the PXRD patterns and IR

Figure 5. TGA curves for 2 and 3.

the anion-exchange induced structural conversions were investigated by IR spectra and PXRD patterns. In particular cases, the anion-exchange induced dimensional changes in the crystalline sample. For instance, when the 3-D net 3 (PF6− form) was immersed in an aqueous solution of NaClO4, the PXRD patterns for the samples taken out from the solution after 3 and 15 h exhibit a decreased intensity for 3 (red squares in Figure 6a) and an increased intensity for 1 (ClO4− form with

Figure 7. (a) PXRD patterns and (b) IR spectra for 1 after the anionexchange with 3 M NaPF6 aqueous solution. The data shown in the top and bottom represent the PXRD patterns and the IR spectra for 1 and 3 prepared by direct synthesis, respectively.

spectra for the samples taken out from the solution after 1 week show decreased peak intensities for 1 (green circles) and the appearance of new peaks for 3 (red squares). After 2 weeks under the same conditions, the peaks for 1 showed weaker intensities but were still present along with increased peak intensity for 3, indicating that the anion-exchange is not completely reversible, as only about 50% of ClO4− are exchanged with PF6−. Since some coordination polymers show the selectivity in the anion-exchange, the above procedures have been extended to other anions. When 2 was also immersed in an aqueous solution of NaClO4, similarly, the PXRD pattern and IR spectrum for each sample collected after 24 h was completely coincident with that for 1, indicating that 2 and 3 with a 3-D structure were each converted to a 1-D structured 1 completely by the exchange of CF3CO2− and PF6−, respectively (Figure S2 in the Supporting Information). However, the ClO4− in 1 did not exchange with CF3CO2− and PF6− even when 1 was immersed in the solution with the chosen anion for 1 week. Similarly, CF3CO2− in 2 underwent complete exchange with PF6− to give 3 within 24 h (Figure S3 in the Supporting Information). While, the PF6− in 3 did not exchange with CF3CO2−. The observed anion-exchange results are summarized in Scheme 3. All products displayed anion-exchange abilities, with ClO4− being preferred over CF3CO2− and PF6−. One the basis of these results, the anion-exchanges show the following order: ClO4− (1) > PF6− (3) > CF3CO2− (2). The observed selectivity for ClO4− does not follow the Hofmeister series,61 which shows the order: PF6− ≫ ClO4− > BF4− > CF3CO2− > NO3− > Cl− > CF3SO3− > HCO2− > H2PO4−. The anion exchange process in coordination polymers also depends on the anion coordination ability as well as the size and shape of the cavity and anions.62−73 The order of crystallographic radii of the anions is as following: PF6− (2.76 Å) > CF3CO2− (2.64 Å) > ClO4− (2.40 Å). As already mentioned, the coordination products showing different structures in this work have very small free

Figure 6. (a) PXRD patterns and (b) IR spectra for 3 after anionexchange with 3 M NaClO4 aqueous solution. The data shown in the top and bottom represent the PXRD patterns and the IR spectra for 3 and 1 prepared by direct synthesis, respectively.

a stairway type 1-D polymer, green circles in Figure 6a) indicating that partial anion exchange had occurred. Furthermore, the PXRD result obtained after 24 h was coincident with that of 1, indicating that the 3-D framework was completely converted to the 1-D polymeric structure 1 by the exchange of PF6− with ClO4−. The IR spectra are also in accord with the above PXRD results (Figure 6b). The IR spectra for the samples taken out from the solution after 3 and 15 h show the gradual disappearance of an intense PF6− peak at 854 cm−1 (red squares) and the growth of the ClO4− peak at 1104 cm−1 (green circles). After 24 h, as expected, the PF6− peak disappears and then IR spectrum does not differ from that of 1, suggesting that the PF6− displaced by the ClO4− and the topology, as stated above, changes from 3-D to 1-D. F



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process for the structural conversion of 3 to 1 exhibiting the dimensional change from 3-D to 1-D. Considering the 3-D framework of 3 with a high symmetric topology and the twisted ribbon structure of 1 shown in the top of Scheme 4, on immersing in an aqueous solution of ClO4−, the crystalline sample of 3 could be rearranged to give a 2-D intermediate via the Ag−S and Ag−N bond breaking. Then, the 2-D intermediate is considered to be converted to the 1-D ribbon via the bond breaking and making followed by the anion coordination resulting in formation of the final perchlorato-type 1-D structure through the dissolution and recrystallization process.74−84

Scheme 3. Structural Transformation of 1−3 by AnionExchange in the Crystalline State

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CONCLUSION Using a flexible bis-cyanobenzyl sulfide ligand (bcbs) incorporating an S-pivot joint, the anion-controlled three silver(I) coordination polymers have been prepared and structurally characterized. In the systematic investigation of the silver(I) coordination networks, it was found that the different coordinating abilities of anions likely contribute to the configurations of the ligand that influence to the topological structures of the resulting frameworks. In the anion-exchange experiments, all silver(I) products without high porosities displayed a selectivity for ClO4− over PF6− or CF3COO− because of its smaller size and moderate coordinating ability in association with its easy entry to bind in the exchange process. Such anion-directed coordination polymers with the flexible ligand and the related anion exchange phenomena accompanying the dimensional conversion in the solid state might be applied to the progress of an essential factor for the design and development of anion-based materials such as anion sensors and ion exchangers. Further works on the organic anions including triflate will be done in due course.

volume which is not sufficient to allow the proper entry and exit of the larger anions. Consequently, the combination of smaller size of ClO4− and its moderate binding affinity to the metal center can easily get entry to show the selectivity in the anion-exchange. To investigate the mechanistic process of the anionexchange, single crystals of 3 in the presence of an aqueous solution of NaClO4 (3 M) were used for atomic force microscopy (AFM).74−77 The anion-exchange procedures in this work show no effect on crystal color, size, and shape. But the sample loses transparency and single crystallinity. The AFM images and height profiles shown in Figure 8 reveal that the crystal surface of the sample undergoes significant transformation, indicating a restructuring of its surface. Within 2 h, for instance, the homogeneous and flat surface of the sample (Figure 8a) became rough, showing holes and clefts (Figure 8b). Then a new crystal phase appears and continues to grow across the surface (Figure 8c). The entire crystal growth process is completed after 48 h resulting in the surface regularly ordered and consists of microcrystallites (Figure 8d). The AFM observations suggest that the present anion-exchange occurs through the typical solvent-mediated transformation,74−84 which forms a new crystalline phase on the surface via the dissolution-crystal growth process. We recently proposed a stepwise mechanistic process for the structural conversion of the coordination networks of a ditopic macrocycle by anion-exchange accompanying the dimensional change from 1-D to 2-D.74 On the basis of the crystal structures, AFM images, PXRD patterns, and IR spectra, as depicted in Scheme 4, we propose a stepwise mechanistic

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EXPERIMENTAL SECTION General. All chemicals and solvents employed in the syntheses were of reagent grade and were used without further purification.The FT-IR spectra were measured with a Nicolet iS10 spectrometer. The ESI-mass spectra were obtained on a Thermo Scientific LCQ Fleet spectrometer. Each product obtained in this work was dried in a vacuum before elemental analysis, which was carried out on a Thermo Scientific Flash 2000 Series elemental analyzer. Thermogravimetric analysis (TGA) was recorded in a TA Instruments TGA-Q50 thermogravimetric analyzer. Samples were heated at a constant rate of 5 °C min−1 from room temperature to 900 °C in a continuous-flow nitrogen atmosphere. The powder X-ray diffraction (PXRD) experiments were performed in a transmission mode with a Bruker GADDS diffractometer equipped

Figure 8. AFM images (top) and height profiles (bottom) of the surface of a crystal of 3 (PF6− form) after immersing in an aqueous solution of NaClO4 (3 M): (a) before anion-exchange, (b) 2 h, (c) 12 h, and (d) 48 h. Image size is 4 × 4 μm in all cases. G



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Scheme 4. Postulated Structural Conversion Process Accompanying the Dimensional Change from 3 (3-D) to 1 (1-D) by Anion-Exchange in the Crystalline State

reduction, and semiempirical absorption correction were carried out using the software package APEX2.85 All of the calculations for the structure determination were carried out using the SHELXTL package.86 In all cases, all nonhydrogen atoms were refined anisotropically and all hydrogen atoms were placed in idealized positions and refined isotropically in a riding manner along with the their respective parent atoms. Relevant crystal data collection and refinement data for the crystal structures of 1−3 are summarized in Table 1.

with graphite monochromated Cu Kα radiation (λ = 1.54073 Å). Caution! Perchlorate salts of metal complexes are potentially explosive and should be handled with great care. Preparation of 1, [Ag(bcbs) (ClO4)]n. Silver(I) perchlorate (11.8 mg, 0.057 mmol) in methanol (2 mL) was added to a solution of bcbs (10.0 mg, 0.038 mmol) in dichloromethane (2 mL). Slow evaporation of the solution afforded a colorless crystalline product 1 suitable for X-ray analysis. mp: 242.5− 244.5 °C. IR (KBr, pellet): 3083, 3048, 2979, 2928, 2248, 1604, 1503, 1418, 1177, 1118 (ClO4−), 1045, 921, 850, 829, 752 cm−1. Anal. Calcd for C16H12AgClN2O4S: C, 40.74; H, 2.56; N, 5.94; S, 6.80. Found: C, 40.51; H, 2.48; N, 5.97; S, 6.48%. Preparation of 2, [Ag2(bcbs) (CF3CO2)2]n. Silver(I) trifluoroacetate (12.6 mg, 0.057 mmol) in methanol (2 mL) was added to a solution of bcbs (10.0 mg, 0.038 mmol) in dichloromethane (2 mL). Slow evaporation of the solution afforded a colorless crystalline product 2 suitable for X-ray analysis. mp: 211.7−213.5 °C. IR (KBr, pellet): 3102, 3003, 2942, 2249, 1682 (CF3CO2−), 1604, 1504, 1417, 1208, 1128, 855, 838, 805, 789, 724 cm − 1 . Anal. Calcd for C20H12Ag2N2O4S: C, 34.02; H, 1.71; N, 3.97; S, 4.54. Found: C, 34.16; H, 1.71; N, 3.97; S, 4.61%. Preparation of 3, {[Ag3(bcbs)4](PF6)3·4CH2Cl2}n. Silver(I) hexafluorophosphate (14.4 mg, 0.057 mmol) in methanol (2 mL) was added to a solution of bcbs (10.0 mg, 0.038 mmol) in dichloromethane (2 mL). Slow evaporation of the solution afforded a colorless crystalline product 3 suitable for X-ray analysis. mp: 234.7−236.2 °C. IR (KBr, pellet): 3101, 3049, 2999, 2930, 2253, 1605, 1504, 1417, 1297, 1252, 1180, 1110, 1 02 1 , 8 3 8 ( P F 6 − ) , 7 4 2 c m − 1 . A n a l . Ca l c d f o r C32H24Ag1.5F9N4P1.5S2: C, 42.33; H, 2.66; N, 6.17; S, 7.06. Found: C, 42.39; H, 2.63; N, 6.12; S, 6.86. Anion-Exchange. The anion-exchange experiments were performed with complex 1−3 (5 mg) by immersing in 3 M aqueous solution (2 mL) of each corresponding sodium salt, and leaving undisturbed at ambient temperature. Each solid sample was collected by filtration, washed several times with water, and then dried in air. The crystalline samples collected from the anion-exchange were not suitable for single-crystal Xray diffraction analysis. So, their structural conversion associated with the anion-exchange was monitored by the PXRD patterns and their IR spectra. X-ray Crystallographic Analysis. All data were collected on a Bruker SMART APEX2 ULTRA diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) generated by a rotating anode. Data collection, data

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01050. PXRD patterns and IR spectra (PDF) X-ray crystallographic files for 1−3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported from NRF (2012R1A4A1027750, 2013R1A2A2A01067771, and 2015R1D1A3A01020410). E.L. acknowledges the support by NRF-2013-Fostering Core Leaders of the Future Basic Science Program. The authors would like to acknowledge Prof. Davide M. Proserpio for helps with the topology analyses including the suggestion on the new topology.

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REFERENCES

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